Internal reference compounds

Figure 9.25—Spectrum of a sample into which an internal standard R has been added. Peak X belongs to the compound to be quantified and peak R to the internal reference compound.

Tetramethylsilane became the established internal reference compound for H NMR because it has a strong, sharp resonance line from its 12 protons, with a chemical shift at low resonance frequency relative to almost all other H resonances. Thus, addition of TMS usually does not interfere with other resonances. Moreover, TMS is quite volatile, hence may easily be removed if recovery of the sample is required. TMS is soluble in most organic solvents but has very low solubility in water and is not generally used as an internal reference in aqueous solutions. Other substances with references close to that of TMS have been employed, and the methyl proton resonance of 2,2-dimethylsilapentane-5-sulfonic acid (DSS) at low concentration has emerged as the reference recommended by IUPAC for aqueous solutions.55 Careful measurements of the DSS-TMS chemical shift difference when both materials are dissolved at low concentration in the same solvent have shown that for DSS 5 = + 0.0173 ppm in water, and 8 = — 0.0246 ppm in dimethyl sulfoxide. Thus, for most purposes, values of 8 measured with respect to TMS or DSS can be used interchangeably. 

The most practical NMR solvent is deuterochloroform (CDQ3) it is relatively cheap and dissolves many different compoionds. Handle this solvent HOOD with care, in the hood, because it is toxic Many other deuterated solvents are commercially available, including acetone, methanol, and water. The universally accepted internal reference compound employed in making these meas-lorementsis tetramethylsilane, Si(CH3)4 (TMS).The most convenient source of TMS is commercially available CDQ3, which contains about 1% TMS for use with CW spectrometers (commercially available 0.03% solutions are more appropriate for FT spectrometers). 

Potentiometric sertsors are very well srrited to perform measurements in real time. For servo control, low respottse times avoid oscillation phenomena. The intrinsic response time is a function of the kinetics of different interfaces and of the electrical properties of the ionic materials (SIC and internal reference compounds). To simplify, one can model a potentiometric sensor by the eqrrivalent electric circuit drawn in Figure 10.16 (this description is very simplified for more details we refer to the specialized literature). Each interface and each material are equivalent to a parallel RC circuit with its time constant x . The transient response is given by... 

Reported LUMO levels of PCBM vary between 3.7 eV and -4.3 eV, because of different measurement methods, either in solution or solid, and different equations used to estimate the values. One of the most convenient teeh-niques to determine the LUMO level of fullerenes is cyclic voltammetry. The LUMO levels of fullerene derivatives can be estimated from the first redue-tion potential. Because reduction potentials depend on the choice of the reference electrode, and vary slightly as a result of different electrochemical cell settings and choice of solvent, the use of an internal reference compound is required for accurate determination of the redox potentials of fullerenes, and any other molecule for that matter. Ferrocene is the most widely used... 

X1.3.1 The preferred internal reference compound for H NMR spectra is tetramethylsilane (TMS). The H t emical shift position for the single H NNHl absorption line observed for this compound is defined as 0.0 ppm. 

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